Digesting process and apparatus



Nov. 22, 1932.

T. L. DUNBAR E'r A1.` 1.888,632

DIGESTING PROCESS AND APPARATUS Filed April 8, 1931 3' Sheets-Sheetf l Nov. 22, 1932. T. L. DUNBAR ET AL 1,888,632

DIGESTING PROCESS AND APPARATUS Filed` April 8, 1931 3 Sheets-Shea?l 2 'Ffzi Nov. 22, 1932.' l T. DUNBAR ET AL 1,888,632

DIGESTING PROCESS AND APPARATUS v Filed April 8, 1931 5 sheets-sheet :s

gmc/"tow Patented Nov. 1932' l -UNITED'STATES PATENT mons Lnomnas DUNBAB AND ALBERT Dennis MERRILL, or WATEBTQWN; NEwYom: v Y

LIGESTING rnocnss AND APPARATUS Application led April 8,

This invention relates to an im roved method' and apparatus for digesting brons material, such as wood chips.

In the digestion of fibrous material, it has' been proposed heretofore that the iuids vent' ed from the digesters during cooking'be used either directly or indirectly for preheating and preconditioning the lfresh acid liquor to be employed in a subsequentcook. In some of these prior proposals, the relief or vented fluids are discharged into one or more headers that are in communication withone or more pressure accumulators containing fresh acid liquor.

One of the objects of the present invention is to combine with such a relief header, means for maintaining a vacuum or sub-atmosf-pheric pressure in the header, whereby the digesters are relieved without building up, any varying back pressures on the digesters. This `facilitates the maintenance' of desired pressures in the digesters during cooking.

Another object is to combine with the accumulator, means for maintaining within the same a substantially constant temperature and super-atmospheric pressure, and to also maintain the liquor within the accumulator at afv substantially uniform acid strength, so

that the preconditioned and preheated liquor pumped into any digester after the latter has been charged with chips, vwill be of substantially the same acid strength and of substantially the same temperature and pressure in each case.y

A further object of the invention is to provide a digesting system by which the fibrous material may be cooked with high strength acids. With the'system, we are able to use cooking acid of a strength as high 4as 18% to 20% free, and may do this with great econ-v omy. By using such high strength acids, it

is practical to reduce the maximum-temperatures in the digester to from 115;J C. to 120 C., and at the same time, to recover the gases which are in the liquor when the digester is emptied. When cooking with such high strength acid, and at frelatively low temperatures, the liquor blown from, the digesters with the-pulp, has a`relatively high SO2 content, and therefore,in our system, the blow- 1931. serial no. 528,693.

pits, vomit stacks and blow-pit relief lineare all of acid proof construction, and are made air-tight so that no gases escape.

With the foregoing objects outlined 5'5"..`

with other objects in view which will appear as the description proceeds, the invention con- 4 sists in thel novel features hereinafter de'- j p scribed in detail, illustratedl in the accoml panying drawings, and more particularly pointed out in the appended claims.

In' the drawings, p v

Fig. 1 is an elevation of one end portion of our novel apparatus.

Fig. 2 is a similar view of the medial portion of the same. 4 y

Fig. 3 is an elevation of the other end portion of the apparatus.

Fig. 4 is an elevation of a suitable type of jet eductor which we use in the apparatus.

In the drawings A, B and C designate digesters, each having a blow-pit, designated respectively A', B and C. Vomit stacks A2, B2 and C2 lead from the blow-pits to a relief header D that conveys blow-pit gases- (a), a valved high pressure relief branch (b) a valved low pressure relief branch and a valved air vent (d). Each digester also has at its lower end a valved cooking liquor entrance branch (e), a valved steam admission branch (f), and a valved blowpit discharge branch (g). The branches (c) discharge into -a secondary relief header F which leads to the acid storage tank G.

Each high pressure branch b) discharges intoI a header H which leads to a jet eductor The side relief branches are also connected with this header by means of valved branches a. The pipes (a) also communicate with another header J by means of valved branches a2, andthe header J discharges into a separator Ki From the separator. waste liquid may liow through a valved pipe (lc) on its way to a sewer,I and in its passage, it can travel through a heat exchanger L for imparting its heat units to fresh acid liquor which also flows through this heat exchanger.

pipe g4 that leads to the jet eductor I. This pipe has a valved branch g5 for feeding fresh liquor into the top of an-accumulator N. The latter has a gauge glass n', and a gas outlet n2 at its top. This outlet communilcates-by a valved branch n3 with al ldown pipe 114 leading from the jet eductor I, and another valved pipe a5 leads from the outlet for discharging gases into the cold liquor line g2. The pipes n3 and n are equipped with back pressure valves n, a7.

The valve 'n is loaded say to discharge gases when the pressurein the accumulator reaches 45 pounds, and the .valve a7. is loaded so as to release gases if the pressure reaches 50 pounds. f v

Pipe n4 leads to a jet eductor O which discharges into a trap pipe P that in turn empties into a jet eductor Q that discharges into a pipe R that leads into the accumulator.

Accumulator liquor may be recirculated V through a valved branch r' that leads from the bottom of the accumulator to a T-connection r2. The T-connection is joined b a pipe r3 and another connection r4 whic is 1n turn connected to a valved pipe '115 that leads to the jet eductor Q'and 1s provided wigi an interposed pump S. i

and t 1s pipe is connectedby a valved conduit f7 to the jet eductor O. When desired,

accumulator liquor may be fed by the pipev 1f( which has an interposed pump T into the jet eductor.

Accumulator liquor may also be fed by the pipe r6 to a valved pipe r8 that discharges into a pump l uor through a valved pipe V in'to a header W that is connected to the liquor inlet branches (e) of the digesters. Y

SO2 may bev vfed from a tank X into the accumulator through avalved conduit as', or it ma be fedfrom this tank into the trap pipe' by means of valved conduit :112. The' tank may have an automatic control valve w3, and the wires :v4 from the electric control will lead into the accumulator to electrode plates, as indicated at c25, so that the strength of ythe liquor in theaccumulator may be employed to open or close the valve m3 for the purpose of automatically yintroducing SO2 either into the accumulator or trap pipe P or both.

Steam, controlled thermostatically-by the ipe r6 leads from the connection r4,

U. vThis pump forces the l1q.

temperature'within the accumulator, may be fed directly into the accumulator by a pipe Y that is connected by a valved branch y to the pipe r?, or the accumulator may be indirectly heated by a coil Z arranged within\ the same. The inlet of this coil is connected by a valved pipe z to the steam pipe Y, and the valved outlet pipe z2 of the coil leads to a steam trap z3. Condensate from this trap may by a pipe a4 into the waste liquor plpe i also impart its heat units to thefresh acid li uor flowing through the heat exchanger L. f eferring now more particularly to Fig. 1 it will be observed that liquid from the bottom of either of the acid towers G42, G3 can flow by vway of valved branches G4, G5 into a pump G6 which forces the same through a pipe G7 that leads tothejeteductor E. This liquid, after absorbing gases and vapors in the eductor, iiows througha down pipe G8, and is forced by a pump G9 through a pipe G10 that is connected to valved so that the heated condensate may branches G11 and G12 which discharge into the tops of the acid towers.

Liquor from the acid tower G3 is ldis'- charged through a valved pipe G13 into a settllng tank G14 that is connected by a valved conduit G15 with a. pump G16 whichl forces the liquor through a pipe G17 into the .top of the tank G1.

A valved conduit G44 leads from the bottom of the settlin tank G14 to the pipe G4,- so that the pump 6 may also draw acld liquor from the settling tank or any other suitL able storage tank and feed it 'to the jet eductor E.:|

It will be observed that the acid storage tanks communicate by means of a pipe G18:

The jet eductors may be of any suitable type, such as shown in Fi 4. For example,

mg cooked with bit-sulphite liquor or the like,

and the digester A has just been filled with f chips from a bin (not shown)y located over the digester. The. cover of the digester A-is clamped on and hot cooking liquor is umped out of the accumulator N throu h -t e pipes r and r8by the pump U, an this ump forces it through the conduit V lan the branch 'pipe (e) into the bottom portion of the digester. This digester is pumped coml pletely full of hot liquor, and during the first portion of the introduction of the'li uor, the valves in thebranches a, b and c are c osed and the valve of the air vent (d) is left open until gas appears, whereupon said valve is closed. The valve in the branch 010 is opened and the gas which arises from the liquor dur ing the continuation of the filling operation is vented lthrough the. branch 010 into the header 011 and-carried by this header to the storage tank G in which they are absorbed by the acid in the tank. The acid in this tank is always maintained at a higher level than the discharge end of the header by means of the overflow conduit G18 which connects the two tanks.

After thev digester'A has been completely filled with the chips and liquor, the pump U is kept running until a liquor pressure equivalent tothe ordinary cooking pressure has been created. Normally, this willv mean that` a pressure from to 80 lbs. will be pumped on the digester, and vat this time the valve in the line 18, the valve in the pipe V, and the valve in the branch (e) will be closed, and

the pump U 'will be stopped. Now, steam is' turned into the bottom of digester A through the branch (f) and during the steam admission, some of the liquor will be relieved from the top of the digester, through the branch (b), and will iow into the header H until a temperature of not less than about 112 C. has been reached, whereupon the valve in the branch (b) is closed, and the valve in the side relief branch (a) is opened, so that this relief liquor will now pass through the-branch a into the main relief header H. A From this header, the relief fluid will flow directly into the jet eductor I, and from the latter it will descend by means of the drop leg n4 and by way of the pipes P and R into the accumulator N. If, when a temperature of 125 C.

is reached in digester A, all the liquor has not been relieved down to the vpoint where the side relief branch (a) is attached to the dgester, and this liquor on analysis is shown to be weak, the valve in the branch ais closed, and the valve in the branch a2 is opened, so that the relief fluid enters the header J and fiows into the separator K.

L The valve in the pipek is then opened, and

any gases coming off from the separator are conducted to the main header H, these gases eventually being led first to the eductor I and thence to the accumulator. The waste weak liquor gathered in the separator will be fed by the line (la) to the heat exchanger L in which it willimpart its heat units to the fresh liquor which is being forced to the jet eductor I by the pump M. In this way, any heat units left in the side relief liquor is imparted to the fresh liquor leaving the storage tanks G. G. l

During the period of the above mentioned operations, digesters B and C relieve dry gas into the header H, and this gas fiows to the jet eductor I where it is contacted with fresh liquor from the storage tank; this fresh liquor being forced under pressure into the jet eductor by means of the pump M. It will be noted that this pump is in continuous operation during cooking, and it acts to vforce i the fresh liquor from the storage tank into the jet eductor I, where the liquor meets the relief fluids from the digesters and condenses them.` From thel jet eductor, the mixture of fresh liquor in somewhat preheated condition, together with therelief gases and liquor is conducted by the drop leg to the jet eductor O. This eductor is operated by liquor leaving the pump T under pressure, and this pressure liquid forces the mixture through the absorbing legs P tothe jet eductor Q which is operated by liquor forced by the pump S. Thepumps S and T draw liquor out of the bottom of the accumulator and feed it to the jet eductors O and Q, due to the fact that at this time, the valve in the pipe '1'8 is closed. Of course, during the lfilling of the next digester that comes on, the valve in the pipe r8must be opened to allow the pump U to force liquor to that digester, butin actual practice, the pipe T8 is of such size thatit will accommodate the pumps S and .'I, and U,

or if preferred, pumps S and T can be conleg n4 very low, which gives us the equivalent of a much higher drop leg, and therefore, by employing the eductors O and Q, we areable to produce a partial vacuum on the header H,

which means that it is vmuch easier to regu-` v'late the flow of the relief gases and liquids from the digesters, as each digester is always relieving against a vacuum or sub-pressure which is kept practically constant by the proper manipulation of the regulating valves of the iet eductors. l l

We have found that the passing of the mixtureof fresh liquor and relief fluids through the circuitous pipe vP causes these fluids to be thoroughlylmixed together beforethey enter the accumulator, and consequently, we are able to maintain a much more uniform acid in the accumulator. However, in order to assure absolute uniformity, we use liquid SO2 under pressure from the tank X. The control `valve of this cylinder is electrically .connected as shown at m4. :c5 with the liquor in the accumulator, and as the conductivity of `the liquor varies with. the strength of the same, the current will control the operation of the motor that takes care of the valve w3.

vTherefore, when the liquor strength drops below a predetermined degree, the motor dioxide to pass through the line into the bottom of the accumulator. thereby increasing the strength of the acid in the accumulator. Just as soon as this strengthl has reached lopens the valve m3 and allows liquid sulphur y i a desired predetermined degree, the current flowing through the plates m5 'will automatically shut off the valve w3, and of course, this will stop the llow of SO2 from ,the tank X to the accumulator.

In the event that the temperature in the accumulator falls below a predetermined degree, the thermostatic control (not shown) may be employed to admit steam from the pipe Y directly into the accumulator through y and r, or into the coil Z. vSteam from ,the coil will flow into-the trap Z3, and the condensate will pass off into the pipe (le) whichieads it through the heat exchanger L, whereby it may give up its heat units to the fresh liquor forced by the pump M. v

y operating as above described, we can maintain the acid strength in the accumulator practically uniform, and we can also maintain the temperature in the accumulator at approximately any desired degree. Normally, in this way, we can maintain the temperature inthe accumulator at about 110 C., and by setting the pop valves n6 and n? so as to maintain a pressure in the accumulator of approximately lbs., we are enabled to maintain the strength and temperature of the acid within narrow limits. We have actually operated with strengths of over 10% free acid. The pop valve a6 is preferably set at about 5 lbs. lower in pressure than the pop valve n7, and consequently, practically all of the overhead gas from the accumulator discharges through the line n3 and the check valve n.33 and a. side eductor 'm14 into the drop leg n4. If there is an excess of overhead gas, the valve a7 acts as a safety devfce and leads the same through the pipe a5 back to the suction side of the pump M after ,it is mixed with the fresh liquor going to the eductor I. I

Referring now to the disposal of the blowpit gases, it will be noted that the pump G6 1s continuously operated, and is pumping weak acid Ii uor outof the tower G2 and feeding this 'quor to the jet eductor" E in which the liquor absorbs any gas or steam that comes o from the blow-pits. From the eductor, this i mixture asses downwardly through the ,drop leg 8 to the pump G9 from which itis forced into the top of the tower G2. The operation of the towers G2 and G3 is reversible, and normally, the o eration is reversed about once a ay.A T e acid tower G2 will be the weak tower one day and the strong tower the next day, while the tower G3 willbe the strong acid tower one day and the weak acid tower the next. This is permitted by the valved connections G11, G12, G4, G5.and G13.

By operating the jet eductor E wefcan produce a partial vacuum on the blow-pit eader D, and consequently, on the vomit stacks, and in this way, we can draw out and which is discharged from the digesters with the pulp when the cooking operation has been completed.

It is not new to condense gases `and vapors from the blow-pits, butso far as we are aware, we are the first to employ a jet eductor for this purpose.- l

' By proceeding as described above, it is practical to use cooking acid of a strength as high as 18 to 20% free, and to do this, with great economy. In this way, we are able to reduce the maximum temperature in the digesters to from 115 C. to' 120 C., and at the same time, We can recover the gases which are in the liquor when .the digester is emptied.

In passing, we may say that when digesting with such high-strength acid at relatively low temperatures, the li uor entering the f' blow-pits is very high in O2 content, and it is economical to recover such SO2.

Another method f operation which we have found very successful is to leave the top of the digester closed after charging the same with -chips, and then, after pumping a pressure of 70 to 80 lbs. on the` liquor in the digester, admit enough steam to maintain this pressure without venting from the top of the digester. If the pressure is heldin this way for about two hours at the start of the cook, a superior and pure cellulose .is produced.

As wood chips normally have a moisture content of from 30 to 60%, the hot acid absorbed by the chips when the digester is pumped causes a drop in pressure after the digester has been completely filled ywith chips and liquor and pumped to the desired degree. It

is believed that the radiation of the heat'froin'v` the shell causes some of the drop in pressure. In any event, it is a fact that after the pump U is stopped with all valves on the digester lclosed, there 'is a perceptible 4dropping in pressure.

It will be observed that the line N10 in Fig. 2 represents the approximate liquor level in the accumulator just before each digester is pumped. We have found in actual. prac.- tice that it is very desirable to so regulate the flow of fresh acid liquor by the regulating valve in the eductor I, that by the time the next digester is ready to be filled,the liquor level in the accumulator will be below the top of the accumulator and preferably at least six feet below the usual top flange: the space unoccupied by the liquor being filled with gases and vapors rising from the hot cooking liquor. We have also found it important to so regulate the valve of the eductor I that at the time the next digester is ready for `pumping, the liquor level in the accumulator will always be brought to about the same elevay tion. Of course, after a digester has been pumped, this liquor level is lowered consider-l ably, normally about h'alf the distance from the line N10 to the bottom ofthe accumu- 55 save all the gases that may be in the liquor lator.

digester is ready for pumping. Each ac cumulator ,should be provided with a gauge 1glasls lnJ for use in maintaining the liquid eve Que `to the spacing of the tol;l liquor level substantially below the top of t e accumulator, there is a large gas space availablein the accumulator above the liquor. We have found that equilibrium as regards the amount of SO2, gas absorbed-by the bi-sulphite liquor is actually to a large extent determined by the gas space above the liquor, Afor with a small gas space, the SO2 gas contained in the liquor will not be proportionate to theA pressure. On the other hand, if the gas space is equivalent in volume to not less'than 25% of the volume of the liquor in the accumulator, equilibrium as regards .the amount ofI SO2 gas absorbed is established.

Each-ofthe eductors I, O, Q, E is bly of the type shown in Fig. 4.

. As an alternative method of operation, the pump S can force liquor from the bottom of preferathe accumulator through the jet educator Q,

. and this will act to draw liquor and gas from the eductor I through a valved pipe N20, di-

rectly into the accumulator without using the.

. absorption legs P. In this modification, the

side relief and vent gases from each digest'er ilow through the header I-I tothe iet eductor I in which they are contacted with a continuously flowing stream of cold fresh liquor from ,the acid storage tanks, and then the mixture of relief gases and cold acid liquor iows through the drop leg n4, N20an`d onv its way down, entrains the gas vented from l the top ofthe accumulator through pipe n3.

The combined mixture ilows to the jet eductor accumulator by the pump S whcli is circuv latnghot liquor from the bottom of the accumulator' through the eductor Q and its connecting pipes. This action of the eductor Q puts a suction on the drop 'leg N20 and thereby lowers the liquid level in the drop leg. l In this way, -we are able to maintain a suction on the main relief header H, whereby the'digesters are always operating against a partial vacuum.

During alena end of the cook, it is sometimes advisable to close the valve in the v branch (b) and'at that time, we then open the valve .infbranclr (c) and vent low pressure Y ing liquor having1 a gases through the header F back to the storage tank. y g v We desire to be understood that the relative dimensions of the parts in the drawings are not accurate, as of course, sizes of parts vary with particular installations.

From the foregoing it is believed that the novel features of ourprocess and apparatus Inay be readily understood by those skilled 1n the art, and we are aware that changes may be made in the details disclosed, without departing from the spirit of the invention, as ex vressed in the claims.

f atis claimed and desired to be secured s by Letters Patent is: 1 s

and at a substantially constant super-atmospheric temperature, and under super-atmos-v v pheric pressure, feeding such liquor from the accumulator into a digester containing fibrous material, and utilizing such liquor in the digester for cooking said fibrous material.

3. In a process, of the character described, maintaining in closed accumulator hot cooksubstantially uniform acid strength and ld at a substantially constant super-atmospheric temperature and super-atmospheric pressure, forcing the liquor in this condition into a digester containing fibrous material, and immediately utilizing the liquor in the digester for pre-cooking sald fibrous material. l'

4. In a process of the character described, maintaining in a closed accumulator coolrin l uor with a strength of over free aci w ile hol gsaid liqlllor in hot condition and under super-atmos eric pressure, feeding the liquor in utilizing e liquor in said digester for pre- Q from whichthe mixture is forced into the ,cooking saidbrous maal' ing liquor of high acid strength at a temperature of about 110 C. and-under a ressure of approximately pounds, fee the liquor in such condition into a digester condition into a dig'ester containinplbrous material, and immediately lool taining fibrous material, and immediately preparing an acid liquor havin a strength of over 10% free acid, bringing t e pressure of said liquor up to approximately pounds and bringing the temperature of the liquor up to about 110 C., then introducing' the liquor in this condition into a digester containing brous material, and utilizing such liquorkin the digester for pre-cooking' said fibrous material.

8. In a process of the character described, charging a digester with fibrous material, then completely filling the remaining space in the digester with high strength acid cooking liquor in hot condition, then forcing this liquor into-the digester to build up a pressure of approximately pounds therein, closing the top of the digester, and then introducing steam into the digester without venting from the top of the latter, and thereby maintaining said pressure for at least an hour at the start of the cook.

9. In a process of the character described, circulating hot liquor A.from an accumulator through a jet eductor, forcing colder acid liquor through a second jet eductor, utilizing the liquor flowing through the second jet eductor to withdraw gases and `vapors from a digester undergoing cooking, and utilizing the liquor flowing through the first eductor for withdrawing the mixture of acid liquor, gases and vapors discharged from the second jeteductor.

10. In a process of the character described, relieving gases and vapors from a digester against sub-atmospheric pressure maintained within a header, forcing relatively cold fresh acid liquor through an eductor and thereby withdrawing the gases and vapors from the header, discharging the mixture of liquor, gases and vapors from the eductor through a descending passageway, discharging the mixture from said passageway into a second jet eductor,`and pumping acid liquor from a pressure accumulator -through the second jet eductor back vto the accumulator for admixing accumulator liquor with the mixture from said passageway and for maintaining suction conditions within the passageway.

11. In a process of the character described,

relieving gases and vapors from a digester` against sub-atmospheric pressure maintained within a header, forcing relatively cold fresh acid liquor through an eductor and thereby withdrawing the gases and vapors from the header, discharging the Amixture of liquor, gases and vapors from the eductor through a descending passageway, discharging the mixture from said passageway into a second jet eductor, pumping acid liquor from a pressure accumulatorthrough the second jet eductor back to the accumulator for adsuction conditions within the pass mixing accumulator liquor with the mixture from said passageway and for maintaining eway, and venting gases and vaporsv from t e top of the accumulator into said passageway.

12. In a processof the character described, relieving gases and vapors from a digester againstsub-atmospheric pressure maintained within aheader, forcing relatively cold fresh acid liquor through an eductor and thereby withdrawin the gases and vapors from the header, disc arging the mixture of liquor, gases and vapors from the eductor through a descending passageway, discharging the mixture from said passageway into a second jet eductor, pumping acid liquor from a pressure accumulator through the second jet eductor back to the accumulator for admixing accumulator liquor with the mixture from said passageway and for maintaining suction conditions within the passageway, and venting vapors and gases from the to of the accumulator into said passageway/W en the ressure in the accumulator rises above a pre etermined degree.

13. In a process of the character described, relieving ases and vapors from af' digester against su -atmospheric pressure maintained within a header, forcing relatively cold fresh acid liquor through an eductor and thereby withdrawing the gases and vapors from the header, discharging-the mixture of liquor, gases and vapors from the eductor through a descending passageway, discharging the mixture from said passageway into a second jet eductor, pumping acid liquor from a pressure' 'accumulator through the second jet eductor back to the accumulator for admixing accumulator liquor with the mixture from said passageway and for maintaining suction con-A ditions'within the passageway, and introducing an extraneous heating agent into the accumulator for heating cooking liquor in the latter. j

14. In a process of the character described, relieving gases and vapors from a digester against sub-atmospheric pressure maintained within a header, forcing relatively cold fresh acid liquor through an eductor and thereby withdrawing the gases and vapors from the header, discharging the mixture of liquor, gases andivapors from thefeductortthrough-.a descending passageway, discharging the mixture from said passageway into a second jet eductor, pumping acid liquor from a pressure accumulator through the second jet eductorl back to the accumulator for admixing accumulatorliquor with the mixture from said passageway,'and for maintaining suction conditions within the passageway, and preheating the fresh acid liquor on its way to the first eductor. f

15. In a process of the character described, relieving gases and vapors from a digester against sub-atmospheric pressure maintained within a header, forcing relatively cold fresh acidliquor through an'educ'tor andthereby l `withdrawing the gases and vapors from the header, discharging the` mixture of liquor,

gases and vapors from the eductor through a descending passageway, discharging the mixture from .said passageway into a second jet eductor, pumping acid liquor from a pressure accumulatorl through the second eductor n' back to the accumulator for admixing accumulator liquor with the mixture from said passageway and for maintaining suction condirelieving gases and vapors from a digester against sub-atmospheric pressure maintained within a header, forcing relatively cold fresh acid liquor Athrough an eductor and thereby withdrawing the gases and vapors from the header, discharging the mixture of liquor, gases and vapors from the eductor through a descending passageway, discharging the mixture from said passageway into a second jet eductor, pumpingacid liquor from a pressure accumulator through. the second jet eductor backto. the accumulator for admixing accumulator liquor with the mixture from said passageway, and utilizing the .condition ofthe li uor in the accumulator for introducing SO2 into the acid liquor.

17. In a process of the character described, relieving gases and 'vapors from a digester against sub-atmospheric pressure main- 'tainel within a. header, forcing relatively cold fresh acid liquor through an eductor and thereby withdrawing the gases and vapors liquor, gases and vapors from the 'eductor through adescending passageway, discharging the mixture from said passagewa into a second jet eductor, pumping acid' iquor from "a pressure accumulator through the second jet ,eductor back to the accumulator for admixing accumulator liquor with the mixture from said passageway and for. maintaining suction conditions within the passageway, and mixing fresh SO2 with the liquor on its way to the accumulator. I

18. In a process of the character described,

I' forcing fresh'acid liquor through a passageway into a pressure accumulator, mixing said liquor in said passageway with vent fluids from Va digester ventina' gases and vapors from the accumulator after a predetermined- .pressure is reached in the accumulator, and

introducing thevent gases and vapors into the liquor in the passageway a ter said liquor has beenv mixed with said vent fluids from a digester.

19. In a process of the character described,

` passing fresh acid liquor through a passageway having an interposed heat exchanger and pump, utilizing waste heat units from a .means for feeding mixed fluids from digesting o eration to heatr said heat exchan er, an discharging preheated acid liquor rom said passageway into a pressure accumulator.

20. In a process ofthe character described PumPmg storage tank through a passageway into a pressure accumulator, utilizing liquor flowing through said passageway to withdraw vent gases and vapors from a digester during a cooking operation, circulating acid liquor resh acid liquor from an acid l from the accumulator `through a portion of said passageway, and utilizing suction crev ated by the circulation of the accumulator liquor for maintaining sub-atmospheric conditions in a portion of said passageway.

21. In a rocess ofthe character described, digesting brous material with relatively high strength acid liquor, dischar ing the vapors from a digester during a cooking o A eration, se arating condensates from t e said condensates vented Hui s and utilizin for preheating the acidv 'quor fed to the accumulator.

23. In a process of the character described,

feeding fresh acid liquor into a pressure ac-A cumulator, introducing steam into said accumulator, and 'utilizing condensates from r said steam for preheating acid liquor flowing to the accumulator.

24. An apparatus of the character described, lcomprising an accumulator, a separator, a conduit for feeding vent fluid from the top of the digester to theseparator, `a

.pressure accumulator, a pipe line for feeding fresh acid liquor to the accumulator,a heat exchan r interposed in said pipe line, and means or feedin hot condensates from the separator throug said heat exchanger.

25. An apparatus of the characterv 4described, comprising a. digester blowiit a; jet eductor, means for forcing -aci `liquor through said jet eductor, a vent conduit con? necting the blow-pit to the jet eductor,- and the iet eductor toan acid tower.

26. An apparatus'4 of the character'des' scribed, comprisng a plurality of acid towers,

a jet eductor, means for" withdra acid liquor from either one of said towers and for forcing the samefthrough said jet eductor, a digesterblow-pit, a conduit for feedin vent gases and vapors from the blow-pit tot e jet" eductor, and means for conveying I fluids fromthe eductor to either one of said acid towers.

27. An apparatus of the character described, comprising a plurality of acid towers, a jet eductor, means for withdrawing" acid liquor from either one of 'said towers and for forcing the same through said jet eductor, a digester blow-pit, a conduit for feeding vent gases and vapors from the blow-pit to the jet eductor, means for conveying mixed fluids from the eductor to either one of said acid towers, and a pump interposed in the last mentioned conveying means for exerting suction on the outlet of the eductor.y

28,. An apparatus of the character de scribed, comprising a digester, an acid liquor storage-tank, a vent outlet for the digester, a valved air vent connected to said outlet, and means for leading cold acid vent through said' outlet to said storage tank.

29. An apparatus of the character described, comprising a pressure accumulator, a passageway for feeding fresh acid liquor to the accumulator, a first jet-,eductor interposed in said passageway, a digester system vent header connected to said jet eductor, ajsecond jet eductor interposed in said passageway between the first eductor and the point where the passageway discharges into the accumulator, and means for pumping acid liquor from the accumulatorthrough said second jet eductor.

30. An apparatus of the character described, comprising a pressure accumulator, a passageway for feeding fresh acid liquor to the accumulator, a first jet eductor interposed in said passageway, a digester system vent header connected to said jet eductor, a second jet eductor interposed in said passageway between the first eductor and the point where the passageway discharges into the accumulator, means for pumping acid liquor from the 'accumulator through said second jet eductor, and means for leading vent gases from the top of the accumulator into said passageway at a point anterior to the first eductor.

31. An apparatus of the character described, comprising a pressure accumulator, a passageway for feeding fresh acid liquor to the accumulator, a' first jet eductor interposed in said passageway, a digester system vent header connected to said jet eductor, a second jet eductor interposed in'said passageway between the first eductor and the point where the passageway discharges into the accumulator, means for pumping acid liquor from the accumulator through said second jet eductor, and means for feeding vent fluids from the top of the accumulator into said passageway at a point posterior to the first eductor. I

32. An apparatus of the character described, ,comprising a pressure accumulator,

a passageway for feeding fresh acid liquor to the accumulator, a first jet eductor interposed in said passageway, a digester system vent header connected to said jet eductor, a second jet eductor interposed in said passageway between the rst eductor and the point where the passageway discharges into the accumulator, means for pumping acid liquor from the accumulator through said second jet eductor, and valve controlled Imeans for feeding SO2 into the accumulator.

' 33. An apparatus 'of the character described, comprising a pressure accumulator,

a passageway for feeding fresh acid liquor to the accumulator, a first jet eductor interposed in said passageway, a digester system vent header connected to said jet eductor, a second 'et eductor interposed in said passageway etween the first eductor and the point Where the passagewayv discharges into the accumulator, means for pumping acid -liquor from the accumulator through said second jet eductor, a'liquid SO2 tank, and valve controlled means for feeding SO2 from said tank into the acid liquor.

34. An apparatus ,of he character described, comprising a pressure accumulator, a passageway for feeding fresh acid liquor to the accumulator, a first jet eductor interposed in said passageway, a digester system vent header connected to said jet eductor, 'a second jet eductor interposed in said passageway between the first eductor and the point where the passageway discharges` into the accumulator, means for pumping acid liquor from theaccumulator through said second jet eductor, and means for heating the interior of said accumulator.

.35. In a process 'of' the character described, maintaining in a closed accumulator hot cooking liquor having a substantially uniform acid strength and held ata substantially constant super-atmospheric temperature and super-atmospheric pressure, and utilizing the strength of the liquor in the accumulator to control the admission of SO2 into said liquor.

' 36. In a process of the character described, maintaining in a closed accumulator hot cookingliquor having a substantially uni,- form acid strength and held at a substantially constant super-atmospheric temperature and super-atmospheric pressure, and utilizing temperature existing in the accumulator to control the admission of a heating agent into said accumulator for heating said liquor.

37. In a process of the character described, maintaining in a closed accumulator, hot cooking liquor having a substantially uniform acid strength and held at a substantially constant super-atmospheric temperature and super-atmospheric pressure, and preventing the liquor in the accumulator from rising above a predetermined level so as to maintain a relatively large gas space in the accumulator above the liquor.

38. The combination with a battery of digesters and means for recovering acid and the heat units thereof when vented from the digesters, of a header operatively connected to the upper ends of the digesters for receiving uids released therefrom, and means for maintainn a substantially constant vacuum in said hea er. v y

In testimony whereof, wehereto" aix our signatures.

THOMAS LEONIDAS DUNBAR. ALBERT DARIUS MERRILL. 

